In the above-identified Ratliff and McBurnett application, a pair of hydraulic motors of the gear type are disclosed which receive operating fluid under pressure from a common source and which are used to operate equipment such as a hoist. Valve means are provided at the inlets of these motors so that under certain conditions of operation, when only one motor is required, the inlet of the other motor is closed so that the gears of that motor operate or turn "starved" and therefore do not consume appreciable prime mover horsepower. In certain of its aspects, the present invention embodies some subject matter disclosed in the prior copending application, in that one of two fixed displacement hydraulic devices of the gear type is provided with inlet valve means which causes this device to be deprived of operating fluid and therefore run dry or starved when not required. In the present invention, this device functions as a vehicle mounted pump which is driven by the vehicle whenever it is in motion. Under certain conditions of operation, as described hereinafter, the pump is deprived of operating fluid so that it operates in a no load condition and does not consume appreciable horsepower of the prime mover.
Various forms of supplemental or emergency hydraulic systems have been provided in the past for operating steering systems, braking systems and the like when there is a failure in the main hydraulic system caused, for example, when the prime mover of a vehicle or other piece of equipment in which the hydraulically operated system is used fails to operate. This invention represents an improvement in such emergency systems and particularly in systems of the kind disclosed in U.S. Pat. No. 3,631,937 issued Jan. 4, 1972 and owned by the Assignee of this application. As indicated in the above-identified patent, in hydraulic power steering systems for vehicles, customary practice is to provide a steering pump driven by the prime mover of the vehicle for supplying operating fluid to a steering valve which operates a steering mechanism in response to movements of a steering wheel or other steering mechanism. If the prime mover acidentally stops while the vehicle is in motion, the primary pump will fail to provide fluid to the steering mechanism. With heavy equipment such a failure can be very dangerous since it is practically impossible to steer the vehicle when the hydraulic system is deprived of operating fluid.
In the system described and claimed in U.S. Pat. No. 3,631,937, a supplemental pump is operated by the vehicle so that the pump is always operated whenever the vehicle is in motion, due to a connection with the vehicle axle or some other part which is always in motion when the vehicle is underway. In this system, the output of this secondary pump is always pumping fluid and thus provides a supplemental source of fluid when the vehicle is in motion. Although this system operates effectively in practice, the pump is always circulating fluid when the vehicle is in motion and therefore consumes an appreciable amount of energy even when the output of the power steering pump is adequate for operation of the power steering system.
The present invention provides a secondary pump which circulates fluid only when actually required, as in a true emergency condition and includes a novel and improved valve system which operates to insure that operating fluid is reliably provided only when it is actually required. At all other times, the pump operates in a no-load state in which it is deprived of operating fluid. Since it is not pumping fluid in that state, it uses very little energy and yet is always available in case of need.
With the foregoing in view, an important object of the invention is the provision of a hydraulic control system for a vehicle driven hydraulic pump, which delivers hydraulic fluid to the pump when the pump is needed.
Another object of the invention is the provision of a hydraulic control system for emergency steering pumps which is reliable and efficient in operation, consuming very little horsepower when the pump is not required.
Still another object of the invention is the provision of a hydraulic control system for a pump which permits operation of the pump only in response to a given condition of operation of a primary hydraulic system.
Still another object of the invention is the provision in a vehicle, of a hydraulic control system for an emergency hydraulic pump which consumes very little energy and operates relatively silently during normal operation of the vehicle.
The above and other objects of the invention are accomplished by an emergency hydraulic system for a steering mechanism or the like which is operated by a primary source of hydraulic operating fluid, the emergency system comprising a pump adapted to be reversibly operated by the vehicle in accordance with its direction of movement and inlet means for the pump comprising a pair of inlet valves connected to pump chambers adapted to be used alternately as suction and discharge chambers depending upon the direction of operation of the pump. The valve means includes devices for preventing the passage of fluid through said inlet means when a primary source of hydraulic fluid is in operation and further includes means communicating with the valve means and responsive to the pressures in said pump chambers for blocking the inlet connected to the pump chamber serving as discharge chamber and for opening the inlet to the chamber serving as suction chamber, so that the pump is supplied with operating fluid at all times when the primary source of fluid is not in operation.